In the search for suitable materials from which to construct high energy density solid state batteries, one of the principal obstacles has been the provision of a suitable electrolyte. A variety of approaches have been tried heretofore. The one which received the most attention among those prior approaches is the one based on polymer solvents in which an optimized amount of ionic salt is dissolved in the polymer solvent (See Armand et al., U.S. Pat. No. 4,303,748; Andre et al., U.S. Pat. No. 4,357,401; and Kronfli et al., U.S. Pat. No. 5,009,970). Other approaches, which possessed both specific advantages and disadvantages, involved glassy solid electrolytes, and certain plastic crystal or disordered crystal electrolytes. Neither of these approaches, nor, indeed, did any of the prior art approaches, obtain all the properties generally recognized as prerequisites to the successful development of a high power solid state battery, namely, (1) high ionic conductivity (about 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 or above); (2) conductivity by lithium cations (to avoid undesirable cell polarization problems); (3) a rubbery consistency (to permit the deformation of the electrolyte as needed to accommodate volume changes during charging and discharging cycles); (4) a wide electrochemical window (to permit the utilization of anode/cathode combinations which provide high voltages); and (5) good adherence to the electrode surfaces (to prevent mechanical/electrical problems that could otherwise develop during charging and discharging cycles).
Each substance heretofore developed for solid electrolyte purposes possesses only a limited number of the above-identified desiderata. None achieved them all. For instance, the so-called superionic glass electrolyte, exemplified in the most successful case by Li.sub.2 S-LiI-Y (where Y is a Lewis acid such as P.sub.2 S.sub.5 B.sub.2 S.sub.3,SiS.sub.2), achieves some of the above listed properties namely, 1,2,4 and 5 but is quite brittle and totally lacks the desired rubbery consistency. Examples of this type of electrolyte are described by Malugani et al. in U.S. Pat. No. 4,331,750 and by Akridge in U.S. Pat. No. 4,585,714.
The prior art salt-in-polymer approach mentioned above, satisfies three of the desiderata namely, 3,4 and 5, but fails miserably with regard to properties 1 and 2. For instance, neither of two recent U.S. Patents dealing with salt-in-polymer electrolytes reported a room temperature conductivity greater than 1.times.10.sup.-5 .OMEGA..sup.-1 cm.sup.-1 for solvent-free or plasticizer-free systems (See: Kronfli et al., U.S. Pat. No. 5,009,970; Knight et al. U.S. Pat. No. 4,737,422). One prior art effort to rectify the poor conductivity of the salt-in-polymer electrolyte involved the addition of low molecular weight plasticizers to the mixture (See: Koksbang et al. J. Power Sources 39, 175, (1990)). However, improved conductivity was achieved at the expense of introducing unwanted volatile components into the electrolyte making the electrolyte susceptible to composition changes when it is exposed to the external atmosphere. Since the solubility of lithium salts in the polymer electrolytes is predicated upon attraction between the lithium cations and the solvating groups in the polymer, these electrolytes further suffer from the fact that the lithium is the less mobile cation. This means that the cation conductivity desideratum, identified as "2" above, is never achieved except in the poorly conducting, single mobile ion polymers which are described by Noda et al. in U.S. Pat. No. 4,844,995. It is believed that it is fundamentally unlikely that this problem can be rectified in the usual polymer/Li salt type of medium. Claims have been made that the problem can be somewhat reduced by using plasticized polymers although no verification of these claims has been found. Examplary salt-in-polymer type electrolytes are disclosed in U.S. Pat. Nos. 4,303,748; 4,357,401; 4,585,714; and 5,009,970.
As is apparent, a great need exists for the development of an improved electrolyte which obtains all of the desiderata listed above without the acquisition of unacceptable deleterious properties. It is toward this goal that the present invention is directed. A preferred embodiment of the present invention obtains all of the desiderata listed above and further provides a predominantly Li.sup.+ -conducting viscous liquid electrolyte suitable for use in polymer sponge or conventional paste electrolytes to obtain a conductivity which is half an order of magnitude higher at room temperature than that obtainable with any previously known polymer-based electrolyte. As will appear, the electrolyte of the present invention obtains an even greater conductivity advantage at temperatures above room temperature.